Zoom lens system

Abstract

A zoom lens system for a compact camera, having a zoom ratio of approximately 2 or 3 and a smaller overall lens length at the telephoto end than the focal distance at the telephoto end, and which optically experiences only a small focus shift due to environmental changes such as temperature or humidity. The zoom lens system includes a positive lens group and a negative lens group. A positive plastic lens element is disposed in either the positive lens group or the negative lens group. The magnification of the system is adjusted by varying .�.the.!. .Iadd.a .Iaddend.distance between the positive and negative lens groups. The zoom lens system satisfies the following conditions:0.9<f.sub.t /f.sub.p <1.6 (1)0.6<m.sub.p '-m.sub.p <1.2 (2).�.2.9<.!. .Iadd.3.13.ltoreq..Iaddend.m.sub.MAX <4.0 (3)where f.sub.t is the focal length of the overall system at the telephoto end, f.sub.p is the focal length of the positive plastic lens element, m.sub.p is the lateral magnification at the telephoto end of the lens group which includes the positive plastic lens element and which is positioned closer to the image than the positive plastic lens element, m.sub.p ' is the lateral magnification at the telephoto end of the lens group which does not include the positive plastic lens element and which is positioned closer to the image than the positive plastic lens element, and m.sub.MAX is the lateral magnification at the telephoto end of the lens group having a maximum magnification.

Description

BACKGROUND OF THE INVENTION

This application is based on and claims priority from Japanese application No. HEI P03-261482 and Japanese application No. P04-96802, the disclosures of which are incorporated by reference herein.

The present invention relates to a zoom lens system which is highly susceptible to temperature and humidity variations. More particularly, the present invention relates to a telephoto-type, two-group or three-group zoom lens system for use with a compact camera, which has a shorter back focus than the zoom lens system for a single-lens reflex camera.

Heretofore, numerous telephoto-type two- or three-group zoom lens systems having a zoom ratio of about 2 or 3 have been known for use with compact cameras. Conventional zoom lens systems have an overall lens length (distance from the first surface to the image plane) at the narrow-angle or telephoto end which is greater than the focal length at the telephoto end.

However, if a zoom lens system whose overall length is greater than its focal length is assembled in a camera, the overall size of the equipment is not sufficiently compact. Thus, a smaller-sized zoom lens system is desired.

Accordingly, each of the lens groups used must be set to have a large power. However, if the power of each lens group is increased, a substantial amount of defocussing (e.g., focus shift) will occur if the lens barrel is even minutely deformed due to temperature or humidity changes. Compact cameras are typically equipped with a range finding optical system separate from the imaging lens system, and the focus adjusting position of the imaging lens system is prefixed with respect to the distance to the object. Therefore, if defocussing as described above results, it cannot be corrected and unfocused pictures are obtained.

SUMMARY OF THE INVENTION

The present invention was designed in view of these problems of the conventional systems. An object of the invention is to provide a compact zoom lens system whose overall lens length at the telephoto end is smaller than its focal distance at the telephoto end and which optically experiences only a small focus shift, e.g., defocussing, in response to an environmental change such as, for example, temperature or humidity.

The zoom lens system of the present invention includes at least two lens groups including a positive lens group and a negative lens group, and has a positive plastic lens element in either the positive lens group or the negative lens group. The system's magnification is adjusted by varying the distance between the positive and negative lens groups. The zoom lens system satisfies the following conditions:

0.9&lt;f.sub.t /f.sub.p &lt;1.6 (1) 0.6&lt;m.sub.p '-m.sub.p &lt;1.2 (2) 2.9&lt;m.sub.MAX &lt;4.0 (3) where f.sub.t is the focal length of the overall system at the telephoto end; f.sub.p is the focal length of the positive plastic lens element; m.sub.p is the lateral magnification at the telephoto end of the lens group which includes the positive plastic lens element and which is positioned closer to the image than the positive plastic lens element; m.sub.p ' is the lateral magnification at the telephoto end of the lens group which does not include the positive plastic lens element and which is positioned closer to the image than the positive plastic lens element; and m.sub.MAX is the lateral magnification at the telephoto end of the lens group having a maximum magnification.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will be more clearly understood from the following description in conjunction with the accompanying drawings, wherein:

FIG. 1 is a simplified cross-sectional view showing the zoom lens system of Example 1 at the wide-angle end;

FIG. 2 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 1 at the wide-angle end;

FIG. 3 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 1 at the intermediate-angle end;

FIG. 4 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 1 at the telephoto end;

FIG. 5 is a simplified cross-sectional view showing the zoom lens system of Example 2 at the wide-angle end;

FIG. 6 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 2 at the wide-angle end;

FIG. 7 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 2 at the intermediate-angle end;

FIG. 8 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 2 at the telephoto end;

FIG. 9 is a simplified cross-sectional view showing the zoom lens system of Example 3 at the wide-angle end;

FIG. 10 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 3 at the wide-angle end;

FIG. 11 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 3 at the intermediate-angle end;

FIG. 12 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 3 at the telephoto end;

FIG. 13 is a simplified cross-sectional view showing the zoom lens system of Example 4 at the wide-angle end;

FIG. 14 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 4 at the wide-angle end;

FIG. 15 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 4 at the intermediate-angle end; and

FIG. 16 is a set of graphs plotting the aberration curves obtained with the zoom lens system of Example 4 at the telephoto end.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENTS

Conditions to be satisfied by the lens system according to the present invention are set forth in the Summary above. Several nonlimiting examples of the present invention are described below and illustrated in the accompanying drawings.

When the temperature or humidity increases, the lens barrel expands to increase the distance between lenses or lens groups housed in the lens barrel. Therefore, in a telephoto-type two- or three-group zoom lens system, the focus will change in a minus direction. Particularly when the overall lens length is shortened, the power of each lens group increases, as does the focus shift. Conversely, if the positive lens element is formed of plastic, the focus will change in a plus direction in response to increased temperature or humidity. Therefore, the focus shift due to expansion or contraction of the lens barrel can be canceled by having a positive plastic lens element in either one of the lens groups in the zoom lens system of the present invention.

If the power of each lens group is increased to shorten the overall lens length, the positive lens element in the positive lens group is prone to having a greater power, and if this positive lens element is formed of plastic, the focus will be overcorrected. Hence, if a plastic lens element is to be included in the positive lens group, a positive plastic lens element solely functioning to compensate for environmental variations must be provided along with the existing positive lens element.

Under these circumstances, the zoom lens systems in accordance with the examples to be described hereunder are composed of either two lens groups (positive and negative) or three lens groups (positive, positive and negative), with a positive plastic lens element being provided in the negative lens group. The positive lens element is advantageously included in the negative lens group since its power can be made comparatively smaller than when the positive lens element is disposed in the positive lens group. Hence, a positive plastic lens element of a small power is preferably used in the negative lens group to ensure that environmental compensation is achieved while simultaneously correcting for chromatic and other aberrations.

If the plastic lens element is positioned closest to the object relative to all the elements in the negative lens group, the diameter of the lens can be reduced, and simultaneously aberrations can be compensated for adequately.

If desired, a positive or negative lens element having a small power may be disposed, fixedly or movably behind the negative lens group.

Condition (1) specifies the ratio of the focal length of the overall system to the focal length of the plastic lens element. If the upper limit of this condition is exceeded, the power of the plastic lens element becomes excessive and the focus shift .DELTA.p.sub.1 due to the focal length variation of the plastic lens element increases.

If the lower limit of condition (1) is not reached, the power of the plastic lens element becomes so small that effectively correcting aberrations within the negative lens group is difficult. Simultaneously, the focus shift .DELTA.p.sub.1 due to the focal length variation of the plastic lens element will decrease, making it impossible to cancel the focus shift .DELTA.p2 due to the variation in the distance between the lens groups resulting from expansion or contraction of the lens barrel.

Conventionally, a plastic lens has been used as a positive lens element in the positive or negative lens group. However, such a positive plastic lens element has a very small power which is less than the lower limit of condition (1), and if the power of each lens group is increased to shorten the overall lens length, it is impossible to cancel the focal variation occurring due to expansion or contraction of the lens barrel.

A positive plastic lens element also has been paired with a negative plastic lens element to cancel the focal variation of the plastic lenses per se. However, the present invention adopts a different approach in which the focal variation of the positive plastic lens element cancels the focal variation resulting from the expansion or contraction of the lens barrel without using a negative plastic lens element.

Condition (2) is associated with condition (1) in specifying directly the defocussing that occurs in the plastic lens element in response to environmental changes. If the upper limit of this condition is exceeded, the defocussing becomes excessive. If the lower limit of condition (2) is not reached, the focus shift .DELTA.p.sub.1 becomes so small that canceling the focus shift .DELTA.p.sub.2 that occurs due to the expansion or contraction of the lens barrel is impossible.

Condition (3) specifies the lateral magnification of the lens group having the highest magnification and which must be satisfied to provide a compact overall system. If the upper limit of this condition is exceeded, the powers of both the first and second lens groups become excessive, greatly varying aberrations upon zooming. If the lower limit of condition (3) is not reached, the power of each lens group becomes so weak that the overall lens length will increase.

Each of the zoom lens systems according to Examples 1-3 described below is composed of a positive first lens group and a negative second lens group, with a positive plastic lens element being positioned closest to the object relative to all elements of the second lens group.

If the zoom lens system is composed of two lens groups, one being positive and the other negative, the following condition (4) is preferably satisfied:

2.9&lt;m.sub.2 &lt;3.7 (4) where m.sub.2 is the lateral magnification of the second lens group at the telephoto end.

Condition (4) specifies the same parameter as does condition (3) except that it is particularly directed to when the overall system is composed of two lens groups.

The zoom lens system according to Example 4 has three lens groups, the first two being positive and the third being negative, and the magnification of the system is adjusted by varying the distance between the first and second lens groups and the distance between the second and third lens groups.

If the zoom lens system has three lens groups, the following condition (5) is preferably satisfied:

3.0&lt;m.sub.3 &lt;4.0 (5) where m.sub.3 is the lateral magnification of the third lens group at the telephoto end.

Condition (5) corresponds to condition (3) and is preferably satisfied to shorten the overall lens length.

Hereunder, the focal variation due to temperature changes is described specifically. If the focal length change due to temperature variations of the plastic lens element is .DELTA.fp, the resulting focus shift .DELTA.p.sub.1 is expressed by:

.DELTA.p.sub.1 =(m.sub.p '-m.sub.p).sup.2 .multidot..DELTA.f.sub.p

In a two-group zoom lens system, the focus shift .DELTA.p.sub.2 due to variations in the distance between lens groups is expressed by:

.DELTA.p.sub.2 =-m.sub.2.sup.2 .multidot..DELTA.dt where m.sub.2 is the lateral magnification of the second lens group at the telephoto end and .DELTA.dt is the change in the distance between the first and second lens groups.

In a three-group zoom lens system, the same parameter is expressed by:

.DELTA.p.sub.2 =-(m.sub.23.sup.2 .multidot..DELTA.dt.sub.12 +m.sub.3.sup.2 .multidot..DELTA.dt.sub.23) where m.sub.23 is the composite lateral magnification second and third lens groups at the telephoto end, m.sub.3 is the lateral magnification of the third lens group at the telephoto end, .DELTA.dt.sub.12 is the change in between the first and second lens groups, and .DELTA.dt.sub.23 is the change in the distance between the second and third lens groups.

The focal length of the plastic lens element changes by approximately +1% when the temperature changes by +30.degree. C. As calculated for the distance between lens groups (.DELTA.dt, .DELTA.dt.sub.12, or .DELTA.dt.sub.23), the lens barrel expands or contracts by approximately +1 to 1.5 .mu.m in response to a temperature change of +1.degree. C.

The plastic lens element in each of the two-group zoom lens systems according to Examples 1-3 has a focal length (f.sub.p) of approximately 50 mm, with lateral magnifications of m.sub.2p (=m.sub.2 =3.2) and m.sub.2p '(=4.1). Given this data, the change in focal length .DELTA.f.sub.p due to a temperature change of +30.degree. C is 0.5 mm and the shifts in focal position .DELTA.p.sub.1 and .DELTA.p.sub.2 will be as follows:

.DELTA.p.sub.1 .TM.0.4 .DELTA.p.sub.2 .TM.-0.3 to -0.5. Thus, the mutual effects of the two focus shifts cancel each other to effectively suppress the defocussing of the overall system.

The data for the three-group lens system shown in Example 4 are: f.sub.p =73.5 mm; m.sub.3p =m.sub.3 =3.56;m.sub.3p '=4.39; and m.sub.23 =2.09. Hence, the shifts in focal positions .DELTA.p.sub.1 and .DELTA.p.sub.2 will be as follows:

.DELTA.p.sub.1 .congruent.0.5 .DELTA.p.sub.2 .congruent.-0.5 to -0.75.

EXAMPLE 1

FIG. 1 illustrates the zoom lens system of Example 1 of the present invention. Specific numerical data for this system are given in Tables 1 and 2. FIG. 4 illustrates graphs plotting the aberration curves obtained with this system at the wide-angle end, the intermediate-angle end and the telephoto end, respectively. In Tables 1 and 2, r denotes the radius of curvature of an individual lens surface, d is the air space between lens surfaces, N is the refractive index, .nu. is the Abbe number, f is the focal length, fB is the back focus, F No. is the aperture ratio, and .omega. is the half-view angle.

TABLE 1______________________________________Surface No. r d N .nu.______________________________________1 12.252 2.15 1.54072 47.22 19.524 1.543 -11.734 1.74 1.84666 23.84 -26.546 0.505 -53.292 2.59 1.53172 48.96 -11.845 0.107 41.762 2.80 1.56732 42.88 -8.050 1.63 1.83400 37.29 -16.685 variable10 -23.722 2.29 1.58547 29.911 -13.547 2.6412 -9.495 1.33 1.77250 49.613 -19.558 1.6614 -10.925 1.44 1.83481 42.715 -23.115______________________________________ TABLE 2______________________________________f 36.10 50.00 68.00fB 10.19 21.54 36.22F No. 1:4.6 1:6.3 1:8.5.omega. 30.3.degree. 23.1.degree. 17.6.degree.d9 7.61 4.91 3.05______________________________________

The tenth surface of the lens system is aspheric. An aspheric surface is expressed by:

X=(CY.sup.2 (1+.sqroot.(1+K)C.sup.2 Y.sup.2)))+A.sub.4 Y.sup.4 +A.sub.6 Y.sup.6 +A.sub.8 Y.sup.8 +A.sub.10 A.sup.10 where X is the distance by which the coordinates at the point on the aspheric surface where the height from the optical axis is Y extend from the tangential plane to the vertex of the aspheric surface; C is the curvature (1/r) of the vertex of the aspheric surface; K is the conic constant; and A.sub.4, A.sub.6, A.sub.8, and A.sub.10 are the aspheric coefficients of the fourth, sixth, eighth and tenth orders, respectively.

The respective values of the conic constant and the aspheric coefficients are listed in Table 3 below. It is noted that the radius of curvature of the aspheric surface shown in Table 1 is that at the vertex of the aspheric surface.

TABLE 3______________________________________ Tenth surface______________________________________ K = 0.00000000 A.sub.4 = 0.76579924 .times. 10.sup.-4 A.sub.6 = 0.18523552 .times. 10.sup.-5 A.sub.8 = -0.10443598 .times. 10.sup.-7 A.sub.10 = 0.00000000______________________________________

EXAMPLE 2

FIG. 5 illustrates the zoom lens system of Example 2 of the present invention. Specific numerical data for this system are given in Tables 4 and 5. FIG. 8 is a graph plotting the aberration curves obtained with this system at the wide-angle end, the intermediate-angle end, and the telephoto end, respectively.

In Example 2, the fourth and tenth surfaces are aspheric and their aspheric coefficients are listed in Table 6.

TABLE 4______________________________________Surface No. r d n .nu.______________________________________1 16.532 1.85 1.53172 48.92 32.843 1.283 -18.916 1.39 1.73077 40.54 -74.920 1.795 -66.046 2.03 1.48749 70.26 -15.222 0.107 -317.259 2.78 1.51633 64.18 -10.337 1.48 1.84666 23.89 -12.937 Variable10 -33.749 2.65 1.58547 29.911 -15.581 2.5912 -11.092 1.33 1.72000 50.313 -38.299 3.2114 -10.954 1.42 1.67790 50.715 -23.249______________________________________ TABLE 5______________________________________f 36.10 50.00 68.00fB 9.21 20.69 35.56F No. 1:4.4 1:6.1 1:8.3.omega. 30.3.degree. 23.1.degree. 17.5.degree.d9 7.93 4.97 2.93______________________________________ TABLE 6______________________________________Fourth Surface Tenth Surface______________________________________K = 0.00000000 K = 0.00000000A.sub.4 = 0.10627284 .times. 10.sup.-3 A.sub.4 = 0.57738292 .times. 10.sup.-4A.sub.6 = 0.20889839 .times. 10.sup.-5 A.sub.6 = -0.12299472 .times. 10.sup.-5A.sub.8 = -0.78624126 .times. 10.sup.-7 A.sub.8 = -0.79635417 .times. 10.sup.-8A.sub.10 = 0.19229557 .times. 10.sup.-8 A.sub.10 = 0.00000000______________________________________

EXAMPLE 3

FIG. 9 illustrates the zoom lens system of Example 3 of the present invention. Numerical data for this system are given in Tables 7 and 8. FIGS. 10-12 are graphs plotting the aberration curves obtained with this system at the wide-angle end, the intermediate-angle end and the telephoto end, respectively.

In Example 3, the fifth and tenth surfaces are aspheric and their aspheric coefficients are listed in Table 9.

TABLE 7______________________________________Surface No. r d n .nu.______________________________________1 14.554 1.91 1.48749 70.22 30.328 1.363 -14.876 1.39 1.83400 37.24 -57.405 1.655 -92.253 2.27 1.58913 61.26 -16.453 0.107 73.857 2.86 1.51454 54.78 -7.967 1.49 1.83400 37.29 -12.334 Variable10 -33.816 2.68 1.58547 29.911 -16.114 1.8912 -12.069 1.33 1.48749 70.213 -37.534 2.9514 -10.954 1.42 1.83481 42.715 -32.374______________________________________ TABLE 8______________________________________f 36.10 50.00 68.00fB 9.97 21.20 35.75F No. 1:4.4 1:6.2 1:8.3.omega. 30.3.degree. 23.1.degree. 17.5.degree.d9 8.60 5.67 3.66______________________________________ TABLE 9______________________________________Fifth surface Tenth Surface______________________________________K = 0.00000000 K = 0.00000000A.sub.4 = -0.52927814 .times. 10.sup.-4 A.sub.4 = 0.72478854 .times. 10.sup.-4A.sub.6 = 0.69987984 .times. 10.sup.-6 A.sub.6 = 0.18865377 .times. 10.sup.-6A.sub.8 = -0.68925287 .times. 10.sup.-7 A.sub.8 = 0.54614578 .times. 10.sup.-8A.sub.10 = 0.23745056 .times. 10.sup.-8 A.sub.10 = 0.00000000______________________________________

EXAMPLE 4

FIG. 13 illustrates the zoom lens system of Example 4 of the present invention. Numerical data for this system are given in Tables 10 and 11. FIGS. 14-16 are graphs plotting the aberration curves obtained with this system at the wide-angle end, the intermediate-angle end and the telephoto end, respectively.

In Example 4, the twelfth and thirteenth surfaces are aspheric and their aspheric coefficients are listed in Table 12.

TABLE 10______________________________________Surface No. r d n .nu.______________________________________1 -32.662 1.30 1.84666 23.82 -50.643 0.203 23.873 3.23 1.51112 60.54 -66.642 variable5 -17.642 1.40 1.83481 42.76 17.642 2.18 1.72825 28.57 -290.344 0.208 35.405 1.50 1.60342 38.09 9.464 3.51 1.62299 58.110 -217.610 0.2011 53.188 2.93 1.58913 61.212 -17.646 variable13 -65.727 2.67 1.58547 29.914 -26.395 2.6115 -10.955 1.50 1.69680 55.516 -768.581______________________________________ TABLE 11______________________________________f 39.10 60.00 102.00fB 11.27 25.45 53.20F No. 1:4.2 1:5.7 1:8.6.omega. 28.3.degree. 19.2.degree. 11.8.degree.d4 3.11 7.55 11.56d12 12.90 7.59 2.81______________________________________ TABLE 12______________________________________Twelfth Surface Thirteenth Surface______________________________________K = 0.00000000 K = -0.10000000 .times. 10A.sub.4 = 0.63008500 .times. 10.sup.-4 A.sub.4 = 0.80292056 .times. 10.sup.-4A.sub.6 = -0.51675400 .times. 10.sup.-7 A.sub.6 = -0.10800984 .times. 10.sup.-7A.sub.8 = 0.10559300 .times. 10.sup.-8 A.sub.8 = 0.40301527 .times. 10.sup.-8A.sub.10 = 0.00000000 A.sub.10 = 0.00000000______________________________________ TABLE 13______________________________________ Ex.1 Ex. 2 Ex. 3 Ex. 4______________________________________f.sub.p 49.8 46.9 49.8 73.5f.sub.r /f.sub.p 1.37 1.45 1.37 1.39m.sub.2 3.28 3.15 3.13 --m.sub.3 -- -- -- 3.56m.sub.p ' - m.sub.p 0.89 1.04 0.96 0.83______________________________________

As described above, the present invention provides a compact zoom lens system by effectively using a plastic lens. Further, in the present invention, as compared to a system having only glass lenses, the focus shift occurring as a result of environmental changes such as, for example, temperature and humidity, is substantially reduced.

Although the present invention has been fully described by way of the examples thereof with reference to the accompanying drawings, various changes and modifications will be apparent to those having skill in this field without department from the scope of the invention as defined in the appended claims.

Claims

1. A zoom lens system, comprising:

a positive lens group and a negative lens group, one of which includes a positive plastic lens element; and

means for adjusting a magnification of said zoom lens system by varying a distance between said positive and negative lens groups, said zoom lens system satisfying the following conditions:

0.9&lt;f.sub.t /f.sub.p &lt;1.6 .�.(1).!.

0.6&lt;m.sub.p '-m.sub.p &lt;1.2

.�.2.9&lt;.!. .Iadd.3.13.ltoreq..Iaddend.m.sub.MAX &lt;4.0

where

f.sub.t is the focal length of the overall system at the telephoto end;

f.sub.p is the focal length of the positive plastic lens element;

m.sub.p is the lateral magnification at the telephoto end of the lens group which includes the positive plastic lens element and which is positioned closer to an image than said positive plastic lens element;

m.sub.p ' is the lateral magnification at the telephoto end of the lens group which does not include the positive plastic lens element and which is positioned closer to the image than said positive plastic lens element; and

m.sub.MAX is the lateral magnification at the telephoto end of the lens group having a maximum magnification.

2. A zoom lens system according to claim 1, wherein said negative lens group includes said positive plastic lens element.

3. A zoom lens system, comprising:

in order from the object side, a positive first lens group and a negative second lens group having a positive plastic lens element; and

means for adjusting a magnification of said zoom lens system by varying a distance between the first and second lens groups, said zoom lens system satisfying the following conditions:

0.9&lt;f.sub.t /f.sub.p &lt;1.6 .�.(1).!.

0.6&lt;m.sub.p '-m.sub.p &lt;1.2 .�.(2).!.

.�.2.9&lt;.!. .Iadd.3.13.ltoreq..Iaddend.m.sub.2 &lt;3.7 .�.(4).!.

where

f.sub.t is the focal length of the overall system at the telephoto end;

f.sub.p is the focal length of the positive plastic lens element;

m.sub.p is the lateral magnification at the telephoto end of the lens group which includes the positive plastic lens element and which is positioned closer to the image than said positive plastic lens element;

m.sub.p ' is the lateral magnification at the telephoto end of the lens group which does not include the positive plastic lens element and which is positioned closer to the image than said positive plastic lens element; and

m.sub.2 is the lateral magnification of the second lens group having a maximum magnification at the telephoto end.

4. A zoom lens system according to claim 3, wherein said positive plastic lens element is positioned closest to the object relative to lens elements forming said second lens group. .Iadd.5. The zoom lens system of claim 1, said zoom lens system consisting of said positive lens group and said negative lens group. .Iaddend..Iadd.6. The zoom lens system of claim 3, said zoom lens system consisting of said first positive lens croup and said second negative lens group. .Iaddend..Iadd.7. A zoom lens system, said zoom lens system consisting of a positive lens group and a negative lens group;

one of positive lens group and said negative lens group including a positive plastic lens element; and

means for adjusting a magnification of said zoom lens system by varying a distance between said positive lens group and said negative lens group, said zoom lens system satisfying the following conditions:

0. 9&lt;f.sub.t /f.sub.p &lt;1.6

0.6&lt;m.sub.p '-m.sub.p &lt;1.2

2.9&lt;m.sub.MAX &lt;4.0

where

f.sub.t is the focal length of the overall system at the telephoto end;

f.sub.p is the focal length of the positive plastic lens element;

m.sub.p is the lateral magnification at the telephoto end of the lens group which includes the positive plastic lens element and which is positioned closer to an image than said positive plastic lens element;

m.sub.p ' is the lateral magnification at the telephoto end of the lens group which does not include the positive plastic lens element and which is positioned closer to the image than said positive plastic lens element; and

m.sub.MAX is the lateral magnification at the telephoto end of the lens group having a maximum magnification. .Iaddend..Iadd.8. The zoom lens system according to claim 7, wherein said negative lens group includes said positive plastic lens element. .Iaddend..Iadd.9. A zoom lens system, said zoom lens system consisting of, in order from an object side, a positive lens group and a negative lens group having a positive plastic lens element; and

means for adjusting a magnification of said zoom lens system by varying a distance between said positive lens group and said negative lens group, said zoom lens system satisfying the following conditions:

0.9&lt;f.sub.t /f.sub.p &lt;1.6

0. 6&lt;m.sub.p '-m.sub.p &lt;1.2

2.9&lt;m.sub.p '&lt;3.7

where

f.sub.t is the focal length of the overall system at the telephoto end;

f.sub.p is the focal length of the positive plastic lens element;

m.sub.p is the lateral magnification at the telephoto end of said negative lens croup which includes the positive plastic lens element and which is positioned closer to an image than said positive plastic lens element;

m.sub.p ' is the lateral magnification at the telephoto end of said positive lens group which does not include the positive plastic lens element and which is positioned closer to the image than said positive plastic lens element; and

m.sub.MAX lateral magnification, at the telephoto end, of said negative lens group having a maximum magnification. .Iaddend..Iadd.10. The zoom lens system according to claim 9, wherein said positive plastic lens element is positioned closest to the object, relative to lens elements forming said negative lens group. .Iaddend.